High-end consumer and professional applications in a wide range of digital image acquisition systems, such as, for example, digital video camcorders, digital still cameras, personal computer video teleconferencing, digital copiers, and infrared image digitizers, require ever better image quality. Several attempts have been made to process signals received from certain sensor devices in such image systems and to provide high resolution output signals.
FIG. 1 is a block diagram illustrating an exemplary digital image acquisition system 100. The system 100 includes an image processing analog front end circuit 110 coupled to a Charge-Coupled Device (CCD) sensor 105 and further coupled to a digital image processing module 120, which includes a digital image processing unit 122, a compression and formatting unit 124, and a media interface 126.
FIG. 2 is a schematic diagram of a prior art image processing analog front end circuit 110 within the digital image acquisition system 100. As illustrated in FIG. 2, the circuit 110 includes an amplifier module 210, such as, for example a Programmable Gain Amplifier (PGA), coupled to an Analog-to-Digital Converter (ADC) module 220.
The PGA 210 is implemented with gain stages to receive an input analog signal 205 from the CCD sensor 105 and to amplify the analog signal 205 to the full scale range of the ADC module 220 in order to obtain an amplified analog signal 215. The ADC module 220 performs analog to digital conversion of the amplified analog signal 215. The ADC module 220 converts the amplified analog signal 215 into a digital output signal 225 through quantization and encoding operations. The ADC module 220 subdivides the range of the amplified analog signal 215 into a finite number of intervals, for example 2n−1 intervals, where “n” represents the number of bits available for a corresponding binary word of each interval, such as, for example, a value between 10 bits and 14 bits. Next, a binary word is assigned to each interval, i.e. to each range of signals, the binary word representing the digital representation of any signal that falls within that interval.
Because an entire interval of the input range is represented by a single digital value, some errors are necessarily present in the conversion operation and are called quantization errors. If a quantization error is different for different independent colors, this quantization error translates into image artifacts, such as, for example, color noise.
FIG. 3 is a diagram illustrating quantization values for separate independent colors. As shown in FIG. 3, considering the analog to digital output granularity as a function of the light intensity, different quantization errors on each independent color, for example the red and blue colors shown, create significant artifacts in the processed image.
In order to avoid such image artifacts, the PGA 210 is used before the ADC module 220 to equalize the relative amplitudes of the input signal for the different colors. This operation is sometimes called white balancing. The white balancing is performed in the digital image processing unit 120 of the image acquisition system 100, but partial color equalization is necessary to be done in the analog domain, prior to the ADC module 220, in order to avoid the image artifacts.
However, the addition of the PGA 210 to the analog front end circuit 110 takes a large portion of the total power dissipation and adds extra thermal noise to the signal path.
Thus, what is needed is an image processing method and analog front end circuit that will increase the resolution of the output signal without a corresponding increase in the power dissipation and noise within the circuit.